The present invention relates to an optical transmission system and an optical node, and more particularly to an optical transmission system and an optical node capable of receiving a transmitted signal at plural nodes. The rapid increase in data traffic, typically over the Internet, is pushing ahead the expansion of transmission capacities of communication networks. Capacity expansion is achieved by converting transmit signals into optical signals and utilizing time division multiplexing or optical wavelength division multiplexing. A transmission device whose capacity is 10 gigabits per second per channel is now available for practical use. A point-to-point wavelength division multiplexing transmission device, which is capable of transmission for a long distance of over hundreds of kilometers by wavelength-multiplexing a few channels to dozens of channels into a single optical fiber using an optical amplifier, regenerator or the like, is now also available for practical use. To meet the increase in demand for greater transmission capacity and the requirements for further economization and service diversification, a ring-shaped optical network annularly connecting communication nodes or a mesh-shaped optical network having meshed connections to increase the flexibility of route selection are under study. The optical transmission system to be used in the ring-shaped optical network is known as an Optical Add-Drop Multiplexer (OADM). The optical transmission system to be used in the mesh-shaped optical network is known as an Optical Cross-Connect (OXC). Such optical networks can be expected to help simplify the operation by the use of a network monitoring and controlling system placing node devices under remote and centralized management or to facilitate end-to-end path management from the start point to the end point of the circuit and increase the speed of path establishment by the mutual linking of the monitoring and controlling units of the node devices.
Further, the use of a configuration in which sophisticated optical transmission technology is used to pass optical signals through nodes as they are, without electrical or optical conversion, enables the whole network to be economically realized. In an optical transmission systems used in such an optical network, one-to-one bi-directional communication is usually accomplished. In OADM and OXC, optical switches are used for the selection of add-drop/through processing or route changing-over of optical signals. At present, known optical switches for such purposes include semiconductor switches and LiNO3 switches utilizing variations in refractive index caused by applying an electric field to the material, Planar Lightwave Circuit (PLC) type switches utilizing variations in refractive index caused by applying heat to the material, movable optical switches in which the position of the optical fiber or the lens is shifted by using an electromagnet, and Micro-Electro-Mechanical Systems (MEMS) type switches in which a micro-mirror fabricated by semiconductor technology utilizing electrostatic power is controlled. There is also known a Wavelength Selective Switch (WSS) which is provided with not only a switch-over function but also a wavelength division multiplexing function using MEMS or liquid crystal technology. WSS is also capable of performing a bridging function to output signals of the same wavelength to two output ports (also known as a drop-and-continue function). A WSS having the bridging function is described in S. Frisken et al. “High performance ‘Drop and Continue’ functionality in a Wavelength Selective Switch”, OFCNFOEC2006 postdeadline PDP14. On the other hand, audiovisual content delivery is discussed as an aspect of increasingly diverse communication services. In delivering audiovisual contents, reception of optical signals transmitted from a given node by N (N>1) nodes can be more economical than audiovisual content delivery using a router. A transmission managing device and a transmission managing method for such one-to-N type communication is disclosed in JP-A No. 2000-031969. According to the invention disclosed in JP-A No. 2000-031969, each of the one-to-N type paths is managed as one of N mutually independent virtual paths. In one-to-one type communication, bi-directional communication in which signals flow in two directions is frequently used, and in that case the type of path used is a bi-directional path. In one-to-N type communication on the other hand, the direction from “1” to “N” being supposed to be the downlink and that from “N” to “1”, the uplink, it is considered that there is a state of one-way communication or traffic transmitted from “1” and received by “N” in which the downlink is heavier. If one-to-N type communication can be achieved with an optical transmission system performing one-to-one type communication as stated above, an efficient network can be architected because paths of different types can be covered by a transmission system of only one type. However, this is regarded as involving the following problem. Namely, since it is possible for optical signals which would otherwise be on the uplink to disappear, an unexpected alarm would be issued. Moreover, pass management is made more complex by the mixed presence of one-to-one type paths and one-to-N type paths.